And still no movement on finally getting Bug 1178764 out the door, despite every other major browser supporting it. Nope, instead they'd rather faff about with the address bar to resounding universal disgust.
Not sure what this has to do with ui changes. What do you think you are going to accomplish by whining about how a project providing free software is not prioritizing your pet feature request?
Well, evidently they implemented backdrop-filter about 8 months ago, but only for the new WebRender stack. And apparently they don't intend to implement it in legacy stacks.
So I guess your feature is tied to the support for WebRender. And that is pretty much limited to some Windows environments, if this article[1] is up-to-date. Both of the Firefox installs I checked don't seem to use it, Linux/X11/Ubuntu and Android (where it's apparently tied to e10s aka Electrolysis, which I could not tell if they even intend to implement for Android any time soon).
If nothing else, I really appreciate the summary, with all the comments and various threads on this it's a bit hard to follow exactly what the hold up is.
It looks like you currently need to go to about:config and enable layout.css.backdrop-filter.enabled. It worked for me without enabling web render. Not that this does much good until it's on by default.